Modern telecommunication systems transmit large amounts of data rapidly between data communications devices. In digital data transmission systems, the receiving equipment must be able to discern where in the bit stream being received from the transmitting equipment one distinct package, i.e., frame, of information ends and where the next frame begins. This is known as frame alignment. Frame alignment is performed both for a new transmission as well as an existing transmission where frame alignment is lost during the course of the transmission.
A commonly used standard for sending telephone signals in the United States is the International Telegraph and Telephone Consultative Committee (CCITT) Recommendation G.704, more commonly referred to as the T1 standard. A T1 system uses a 1.544 megabits/second pulse code modulation (PCM) digital signal. The T1 signal includes 24 time domain multiplexed channels, for carrying 24 separate channels, such as voice conversations, simultaneously on the single T1 carrier. In the T1 standard, eight bits comprise a single sample composed of one 7-bit AID sample of one voice channel plus one signaling bit. A “frame” is defined as 24 samples, plus one framing bit termed the F bit which occupies the first bit position, for 193 total bits per frame. A superframe is defined as 12 frames. Each channel is sampled at an 8 KHz rate, so the T1 signal comprises 8000 193-bit frames per second, or 1.544 megabits/second.
In a data transmission system such as a T1 system, a frame alignment unit, i.e., frame aligner, within a data receiver searches the incoming bit stream for a framing detection pattern. When the frame aligner identifies the framing pattern, the unit declares a frame alignment condition.
In order to prevent framing onto a false framing pattern, a conventional frame aligner only declares frame alignment when there are no false framing patterns present. This is because the conventional frame alignment algorithm employed by a frame aligner is not capable of discerning the difference between a false framing pattern and the true framing pattern. Thus, conventional frame alignment units wait until the false framing patterns go away and there is only one frame detection candidate left. The remaining candidate is then chosen as the frame alignment position. During the wait, new false framing patterns may appear and these must also go away before a frame alignment position can be chosen. In a conventional frame alignment algorithm, the minimum false framing pattern length, i.e., false framing pattern threshold, is fixed. That is, such frame alignment algorithms maintain a fixed false framing pattern threshold regardless of how many frames have passed while the algorithm waits for the false framing patterns to go away, i.e., fall below the fixed false framing pattern threshold. However, setting fixed thresholds extends the time necessary to find frame alignment. New false framing patterns may form while old false framing patterns disappear preventing frame alignment from being found.
Therefore, there is a need for methods and apparatus for performing frame alignment which eliminate or at least reduce the effects associated with the shortcomings of the prior art as discussed above and which otherwise exist in the art.